The present invention relates to molecular silylalkylboranes, a process for their preparation, oligo- or polyborocarbosilazanes, a process for their preparation and their use and silicon carbonitride ceramics and a process for their preparation.
The process for producing multinary, non-oxidic ceramics via molecular single-component precursors has achieved outstanding importance. It has opened up the path to nitridic, carbidic and carbonitridic systems which are not accessible via conventional solid-state reactions. The products are distinguished by high purity, homogeneous distribution of elements and uniform particle size.
Materials which consist of Si, B and N and, if appropriate, also C, demonstrate particular properties with respect to thermal stability and resistance to oxidation. They can be used as bulk material or else for coatings and as fibrous material. The boron-containing materials exhibit an increased inhibition of crystallization, whereas the carbon-containing ceramics have, in addition, higher decomposition temperatures than carbon-free ceramics.
According to U.S. Pat. No. 5,233,066, synthesis of the amorphous ceramics Si.sub.3 B.sub.3 N.sub.7 and SiBN.sub.3 C from the precursor trichlorosilylaminodichloroborane (TADB) is achieved by crosslinking with ammonia or amines and subsequent pyrolysis in a gas stream.
In this method, the composition of the products is set firstly by the single-component precursor and secondly by the type of crosslinking. Whereas the Si:B ratio is set by the precursor to 1:1, the N:C ratio is a function of the choice of methylamine as crosslinking reagent.
The carbon is incorporated into the ceramics here via the organic amine side chain. However, this takes place in an uncontrolled manner via a reaction which has not been described in detail. The disadvantage of this procedure is the low possibility of varying the C content. Extending the side chain does not lead inevitably to a higher C content in the ceramics, but to graphite deposits in the material. This has an adverse effect on the properties. In addition, a large part of the carbon is lost during the pyrolysis, since the side chains are eliminated in the form of volatile alkanes, alkenes etc.
According to Appl. Organomet. Chem. 10 (1996) 241-256, polymeric boron-containing carbosilanes are obtained by reacting vinyl-containing polysilanes or polysilazanes with boron adducts of the formula BH.sub.3 .multidot.SR.sub.2 where R.dbd.C.sub.1 -C.sub.18 -alkyl.
According to DE-A 43 20 785, trissilylalkylboranes of the formula B[--C.sub.2 H.sub.4 --SiCl.sub.2 X].sub.3 are obtained by reacting vinylsilanes of the formula CH.sub.2 .dbd.CH--SiCl.sub.2 X with BH.sub.3 .multidot.THF.
In these methods, either polymers are obtained directly or the ratio of Si to B does not correspond to the ideal ratio of 1:1.
The object of the present invention was therefore the provision of novel precursors which are simple to prepare and provide an opportunity for the controlled introduction of carbon, without having the disadvantages of the prior art.